Composed performance

A high-strength aluminium alloy prepared by researchers in the US could significantly improve the performance of next-generation fighter aircraft, specifically the F-35 Joint Strike Fighter.

Developed at AMES, the US Department of Defence laboratory run by IowaStateUniversity, the aluminium-yttrium-nickel-cobalt alloy has the potential to replace heavier and costlier components of ‘cool’ sections of jet engines, such as in fan blades. The material could also be used in other sections of the aircraft, including wing spars.

Larry Jones, director of the Materials Preparation Centre (MPC) at AMES, explained: ‘The hope is that the lighter alloy would essentially reduce engine weight, so we could increase the plane’s efficiency or passenger carrying capability.’

Project partners Pratt & Whitney estimate that replacing jet engine components with the Al-Y-Ni alloy could reduce overall engine weight by 350lb. Jones pointed out that traditionally a reduction of just a few pounds in aircraft weight is considered an achievement.

Jones believes that other engine and airframe manufacturers would be interested in using the material once it proves successful. ‘It is one of the materials they are envisioning to replace titanium in different aspects of the frame. It has sufficient strength that they can use it to replace more expensive titanium.’

According to Jones, Boeing has said it would be interested in using it as a replacement for titanium wing spars, while Pratt & Whitney engineers are developing fan blades with the alloy and looking to trial them in the next six to 12 months.

The alloy is produced via high-pressure gas atomisation (HPGA), which uses a special nozzle to blast a stream of molten alloy material with a pressurised gas such as helium or nitrogen. The result is powder-fine metal particles that are highly uniform in chemical composition and, because they cool so quickly, exhibit the amorphous structure of the liquid metal rather than the crystal structure normally found in bulk metals.

Once completed, the powdered metal is vacuum hot-pressed and hot-extruded into a finished product. This bonds the particles together and the partly amorphous, partly crystallised structure gives HPGA-produced materials the improved strength and ductility properties. Initial tests of the alloy reveal a tensile strength of 100,000psi, whereas the top commercial aircraft-grade aluminium is just 70,000psi.

However, Jones said that currently they have not been able to identify a commercial vendor that can replicate the powder as commercial manufacturers are having problems recreating lab conditions for the process.

Tests of the Al-Y-Ni alloy produced by a commercial manufacturer have yielded results in the 90,000-92,000psi range and this is due to a number of inherent problems that affect the strength.

‘Aluminum powders are highly explosive,’ Jones said. ‘By using nitrogen gas it creates a nitride passivition layer so the powders are less likely to be explosive. This nitride layer breaks down during sintering, resulting in very strong bonds between the particles.’

However, the commercial process injects oxygen into the atomisation gas stream to create a controlled oxidation of the powders. This reduces explosiveness but Jones says any exogenous material will result in a weaker end product and that includes any oxidation that takes place.

To counter this problem, the material being produced by the MPC will be kept in an inert environment until after the vacuum hot-pressing process is completed.

The MPC has modified its HPGA system to capture the powder in a container under an inert atmosphere. The powder will be sieved to less than 32 microns in size in an inert atmosphere glove box before being shipped in a sealed container to DWA Aluminum Composites, Los Angeles, where the vacuum hot-pressing will be completed. After vacuum hot-pressing the pressed and sintered powder billet will be extruded. Only then will it be exposed to the normal atmosphere.

The MPC will study the results and modify the process to improve processing at the commercial level.